Structural materials for use typically in automobiles and other transport equipment and in industrial machinery secure strength and toughness at certain levels by forming a steel for heat treatment supplied by a steel maker into a predetermined shape typically through press forming, and subjecting the formed steel to a heat treatment such as quenching and/or tempering. For providing a structural material having an especially high strength and high toughness, a technique has been proposed in which the steel is controlled to have a prior austenite (γ) grain size of 5 μm or less.
Typically, Patent Literature 1 (PTL 1) discloses a high-strength steel member which contains C: 0.25 to 0.35 percent by mass, Si: 0.5 percent by mass or less, Mn: 0.2 to 1.0 percent by mass, P: 0.01 percent by mass or less, S: 0.01 percent by mass or less, Al: 0.01 to 0.1 percent by mass, N: 0.002 to 0.01 percent by mass and Ni: 7 to 12 percent by mass, with the residue being Fe and inevitable impurities or which contains C: 0.25 to 0.35 percent by mass, Si: 0.5 percent by mass or less, Mn: 0.2 to 1.0 percent by mass, P: 0.01 percent by mass or less, S: 0.01 percent by mass or less, Al: 0.01 to 0.1 percent by mass, N: 0.002 to 0.01 percent by mass and Ni: 7 to 12 percent by mass and further contains one or more elements selected from the group consisting of Cr: 0.1 to 1.0 percent by mass, Mo: 0.01 to 1 percent by mass, Ti: 0.01 to 0.05 percent by mass, Nb: 0.01 to 0.05 percent by mass and B: 0.0003 to 0.005 percent by mass, with the residue being Fe and inevitable impurities. This high-strength steel member includes very fine grains of prior austenite with grain sizes of 5 μm or less, has a tensile strength of 1400 MPa or more and excels in resistance to delayed fracture.
PTL 1 discloses a process which includes the step of heating the steel to a temperature of 850° C. to 1000° C. thereby hot-working the steel, performing a finish working on the steel in a temperature range of 700° C. or lower and the martensite start point (Ms point) or higher at a reduction of area of 20 to 50 percent by mass and immediately cooling the steel; and the step of performing rapid heating of the steel as a subsequent heat treatment at a temperature equal to or higher than the Ac3 transformation point and equal to or lower than 900° C., and immediately cooling the heated steel.
Patent Literature 2 (PTL 2), for example, discloses a high-strength steel excellent in resistance to delayed fracture, which has a specific composition, has been quenched and tempered under specific conditions and has an austenite grain size in terms of ASTM No. 8.5 or more.
PTL 2 describes that the steel is heated to a temperature equal to or higher than the Ac3 transformation point, quenched and then tempered at a temperature equal to or higher than 580° C. and equal to or lower than the Ac1 transformation point under such conditions that PLN is 16.8×103 or more.
Independently for example, Patent Literature 3 (PTL 3) discloses a spring steel wire having a tempered martensite microstructure obtained from a specific composition mainly through quenching and tempering, in which the martensite grains include carbides having such shapes as to have an average aspect ratio of 3.0 or more.
PTL 3 describes that the heating in quenching and tempering is performed at a heating rate of 50° C. to 2000° C. per second for a holding time of 0.5 to 30 seconds.